Epitaxial CoSi.sub.2 films have been grown on Si(111) substrates under ultra-high vacuum conditions using a variety of growth techniques. The most widely used technique is that of solid phase reactive epitaxy (SPRE), in which a pure cobalt layer is deposited at room temperature onto the substrate and then annealed at an elevated temperature.
In addition, the techniques of molecular beam epitaxy (MBE) and reactive deposition epitaxy (RDE) have been examined. In the foregoing methods, either pure cobalt is deposited (the latter technique) or cobalt and silicon atoms are codeposited in stoichiometric ratio (the former technique) onto a silicon substrate held at an elevated temperature.
Thin epitaxial CoSi.sub.2 films, about 1 to 100 nm thick, formed by these techniques generally have a high density (&gt;10.sup.7 cm.sup.-2) of pinholes in the silicide layer, which results in an electrical shorting problem for multilayer structures.
It is known that a modified SPRE technique which utilizes the deposition of an amorphous silicon (a-Si) capping layer following the deposition of pure cobalt prior to annealing helps to reduce the size and density of pinholes to approximately 5.times.10.sup.6 cm.sup.-2. In the process disclosed herein, a modified solid-phase epitaxy (SPE) technique is used to produce CoSi.sub.2 layers on Si(111) which are pinhole-free within a detection limit of 10.sup.3 cm.sup.-2. In contrast to the deposition of pure cobalt used in the modified SPRE technique of the prior art, the modified SPE technique disclosed herein uses the room temperature codeposition of cobalt and silicon in the stoichiometric ratio of 1:2, followed by the deposition of an a-Si capping layer.
The growth of so-called pinhole-free CoSi.sub.2 films has been reported, but the pinhole detection limit is generally not specified. The techniques of scanning electron microscopy and transmission electron microscopy have been used in these reports to determine the pinhole density, and these techniques are typically limited to a detection level of about 10.sup.5 pinholes per cm.sup.2. Therefore, the term "pinhole-free" CoSi.sub.2 has, in the past, been used to describe CoSi.sub.2 films with pinhole densities less than or equal to .apprxeq.10.sup.5 pinholes per cm.sup.2. In the present disclosure, pinhole-free CoSi.sub.2 refers to a film with pinhole density less than 10.sup.3 per cm.sup.2.
Recently, Henz et al, Solid State Communications, Vol. 63, pp. 445-449 (1987) reported a pinhole-free SPE technique employing the stoichiometric codeposition of cobalt and silicon at room temperature, followed by appropriate annealing (.ltoreq.450.degree. C). No a-Si capping layer was used in between the codeposition and the annealing. The low annealing temperature is required for the pinhole-free growth, since the present inventors observed pinholes with densities of 10.sup.7 to 10.sup.8 cm.sup.-2 in CoSi.sub.2 films using this technique, but annealed at temperatures higher than 500.degree. C. The low annealing temperature of .gtoreq.450.degree. C. required for pinhole-free growth in the technique of Henz et al results in CoSi.sub.2 films of lower crystalline quality, as evidenced in results by the present inventors using Rutherford backscattering spectroscopy in the channeling mode. Furthermore, CoSi.sub.2 films annealed at low temperature have higher resistivity than films annealed at higher temperatures. A CoSi.sub.2 film with a high resistivity is less desirable for device applications.
One possible mechanism for silicide pinhole formation is the high surface energy of CoSi.sub.2 (111) relative to Si(111). By exposing the underlying silicon surface through the formation of pinholes in the CoSi.sub.2 film, the total energy of the epitaxial system can be reduced. One approach to reduce the total energy without pinhole formation is to cover the high energy CoSi.sub.2 surface with a silicon cap. However, the prior art a-Si cap technique does not achieve pinhole-free growth, since the a-Si cap is consumed during the silicide formation and the CoSi.sub.2 surface is exposed.
Thus, a process is required for the formation of pinhole-free CoSi.sub.2 films on Si(111) while retaining high crystalline quality of the subsequent epitaxial growth.